"Any way to fix this?"
Not sure if you can "fix" it, as it may be some kind of conflict between the drive's controller or firmware that prevents it from being seen or "waking up" after the Mac has gone to sleep for a while.
It's possible the drive itself -- upon seeing that the Mac has gone to sleep -- goes to sleep, too. But then, when the Mac wakes up, the drive keeps right on sleeping (and hence, no longer has a "presence" on the firewire bus).
A suggestion that may or may not work:
- After the computer has been sleeping, if you can't "see" the drive mounted after the wakeup, open Disk Utility
- In DU, on the left-hand side try clicking the icon for the drive, then go up and click the "mount" button. Does that help?
Back in the days of OS 9 and SCSI, there was a very handy tool called "SCSI Probe" that could mount drives on the SCSI bus. But unfortunately, no one has ever seen fit to introduce a "Firewire Probe" utility that offers similar functionality for OS X and firewire.
If you can't "revive" the drive with Disk Utility (as above), perhaps the only workable "solution" may be to physically disconnect and re-connect the drive in this situation.