I have a computer running Squeeze that now needs to read exFat formatted drives. This doesn't seem possible with Squeeze -- although I could be in error here. The computer runs an LTO5 tape drive. The issue now is I need a drive that works on OSX and on this Debian-powered machine and exFAT seems the best bet. So, here's two questions:

1. It is possible to get Squeeze to read exFat? (backports, etc).

or

2. Better to upgrade to Bullseye? And if so, what is the best path to go from Squeeze (6.0) to Bullseye (11.0)?

It was tricky to get the LTO5 software installed and compiled to run that drive. Its a workhorse for us -- and that is ALL this computer must do -- and so we never wanted to mess with it. . I suppose when upgrading we could mess this up, then again we really should be keeping the machine up to date with stable Debian.

The best way to go from 6 to 11 is a new install. IMHO it would be time wasted to try the dist-upgrade process.
Make sure you backup everything that is important. If the LTO5 software is something you didn't write yourself then getting it to work may take some effort with newer kernels/libraries.

If this is something really critical I would suggest migrating to a newer PC with similar hardware if possible in a "lab" environment so that you can iron out the problems.

exFAT support as FUSE has been around since 2009 and 6 was released in 2011 so it is possible the included FUSE driver works ok but there could be problems. There is always FAT32 but it depends on your file sizes.

http://archive.debian.org/

No Debian expert here but my gut reaction is don't update - reinstall.

The reason is that programs are dropped and new ones added, etc. If it's a workhorse, it's probably past it's "Best Before" date so you might want another HD, a ram upgrade or other things to keep it singing for another few years.

If it's a matter of cost, calculate the cost of downtime, i.e. not having it. That's the cost you need to consider.

Yeah, best course here might be to format a new machine, set up LTO5 … and thus have the old machine untouched so it can run the LTO5 drive while new machine is tweaked and tuned … so no down time.

Too many changes - install afresh.

Ensure this machine is not connected to the internet, and put another machine which you will be able to support, with Debian or whatever. It can be old laptop, or anything and connect it to legacy machine over one dedicated segment cable. On the new machine
you will be free to do everything to support new requests. If it is important to support Mac world, maybe this machine should be Mac as well. I wouldn't touch legacy, but would do everything to replace it.