Setting aside the internal connector, most Mac laptops (and all modern Macs) have a Target Mode[1] that exposes the internal hard drive externally as a SCSI, FireWire, Thunderbolt, or USB device you can mount on another computer.
In this case, it's a PowerBook Duo with a SCSI disk, so you'd need a Duo Dock or similar[2], a HDI-30-to-something-a-little-more-standard adapter [3], and whatever combination of cables, converters, and controllers are required to connect a parallel SCSI device into a more modern system.
For example, if you have a machine with a PCI Express slot (or Thunderbolt PCI Express enclosure), I'd recommend any of the LSI Logic Ultra320 adapters available on eBay for < $50, as drivers are readily available for macOS (AppleLSIFusionMPT.kext, included in all Mac OS versions worth mentioning, up to and including Sonoma, Intel and Apple Silicon), Linux (in-kernel mptspi driver), and Windows (the Windows Server 2008 driver[4] should work on current Windows versions).
Assuming all these things are in order, connect the Macs with a SCSI cable, hold down T while booting the PowerBook to enter target disk mode, and answer no to any prompts your OS may display offering to format the unreadable disk you just attached!
As for accessing the HFS filesystem, assuming macOS, my preferred method would be:
(use the "diskutil list" command to find <Number>)
2. Shut down the PowerBook.
3. Make a backup copy of the image in case something goes wrong.
4. Install Mac OS 9 in qemu[5] and attach the image.
5. Use Alsoft PlusMaker[6] in Mac OS 9 to do an in-place conversion of the filesystem on the image from HFS to HFS+ (download the first file, use StuffIt[7] to extract the file and Toast[8] to mount the image, and run PlusMaker directly from the image to avoid being prompted for a serial number).
6. Shut down Mac OS 9 ("Shut Down" is in the Mac OS 9 Finder's "Special" menu).
7. Mount the resulting HFS+ image in (current) macOS (double-click in Finder, "hdiutil attach <Name>.img" from a command prompt, etc.).
Other options exist, but this is the easiest full-fidelity one that comes to mind.
As a bonus, you'll now have an emulated Mac OS 9 system that should run most of the applications from the original PowerBook's hard drive on your modern Mac.
In this case, it's a PowerBook Duo with a SCSI disk, so you'd need a Duo Dock or similar[2], a HDI-30-to-something-a-little-more-standard adapter [3], and whatever combination of cables, converters, and controllers are required to connect a parallel SCSI device into a more modern system.
For example, if you have a machine with a PCI Express slot (or Thunderbolt PCI Express enclosure), I'd recommend any of the LSI Logic Ultra320 adapters available on eBay for < $50, as drivers are readily available for macOS (AppleLSIFusionMPT.kext, included in all Mac OS versions worth mentioning, up to and including Sonoma, Intel and Apple Silicon), Linux (in-kernel mptspi driver), and Windows (the Windows Server 2008 driver[4] should work on current Windows versions).
Assuming all these things are in order, connect the Macs with a SCSI cable, hold down T while booting the PowerBook to enter target disk mode, and answer no to any prompts your OS may display offering to format the unreadable disk you just attached!
As for accessing the HFS filesystem, assuming macOS, my preferred method would be:
1. Create a raw disk image:
(use the "diskutil list" command to find <Number>)2. Shut down the PowerBook.
3. Make a backup copy of the image in case something goes wrong.
4. Install Mac OS 9 in qemu[5] and attach the image.
5. Use Alsoft PlusMaker[6] in Mac OS 9 to do an in-place conversion of the filesystem on the image from HFS to HFS+ (download the first file, use StuffIt[7] to extract the file and Toast[8] to mount the image, and run PlusMaker directly from the image to avoid being prompted for a serial number).
6. Shut down Mac OS 9 ("Shut Down" is in the Mac OS 9 Finder's "Special" menu).
7. Mount the resulting HFS+ image in (current) macOS (double-click in Finder, "hdiutil attach <Name>.img" from a command prompt, etc.).
Other options exist, but this is the easiest full-fidelity one that comes to mind.
As a bonus, you'll now have an emulated Mac OS 9 system that should run most of the applications from the original PowerBook's hard drive on your modern Mac.
[1] https://developer.apple.com/library/archive/technotes/tn/tn1...
[2] https://en.wikipedia.org/wiki/PowerBook_Duo#Docking_stations
[3] https://www.amazon.com/dp/B0081SAIS2/
[4] https://www.broadcom.com/support/download-search?pg=Legacy+P...
[5] https://wiki.qemu.org/Documentation/GuestOperatingSystems/Ma...
[6] https://www.macintoshrepository.org/19-plusoptimizer-plusmak...
[7] https://www.macintoshrepository.org/809-stuffit-deluxe-1-5-x...
[8] https://www.macintoshrepository.org/1724-toast-5-titanium