Zyxel ZyWall USG50 - doesn't reliably boot (boots maybe 25% of the time), can't seem to update firmware - can this be fixed or should I toss it?

I got a ZyWall USG50 for dirt cheap (so I won't be disappointed if I just have to recycle this), but I'd like to get it to work if possible. Here's what's happening (I have it hooked up via serial port to monitor the console):

• Plug in power from cold state.
• Boot 01: Segmentation fault, froze at App. Patrol Daemon. Failure to boot.
  • Unplug, wait five seconds, replug.
• Boot 02: LOTS of file errors. Boots successfully.
  • Log in, reboot via reboot command
• Boot 03: LOTS of file errors. Boots successfully.
  • Log in, shutdown via shutdown command. Unplug, wait a while, replug
• Boot 04: zysh terminates by segfault. Failure to boot.
  • Unplug, wait five seconds, replug.
• Boot 05: Segmentation fault, froze at App. Patrol Daemon. Failure to boot.
  • Unplug, wait five seconds, replug.
• Boot 06: Froze at App. Patrol Daemon, no segfault. Failure to boot.
  • Unplug, wait five seconds, replug.
• Boot 07: System hang, automatically reboot
• Boot 08: Freezes at ZyWall system daemon...

There seem to be a lots of different errors: of the times it does not boot, only one of the conditions is repeated. When I try to upload the firmware via console and ftp, I get a message about invalid firmware format (and I've tried all three .bin files on the Zyxel website).

Any ideas for how to salvage this? I'd like to get it into a working state (where it will at least reliably boot most of the time), but I'd also be happy if someone tells me there's a way to put a custom OS on it. If I can get it to work with some functionality I'll get use out of it. Thanks. (Sorry if this isn't the right subreddit for this.)