As of late, I've been running my MBP in clamshell mode. Been working really nice, actually, except when turning on my external FW drives. They won't mount unless I reboot. When unmounting and then truning off and back on again, a reboot is required to mount. I don't remember this happening except when going to clamshell mode. Any reason that I should suspect?
I knew I should've included more info. I have them powered. I had tried removing the FW cable and reconnecting with no success. Just turned on my drive now and it mounted, so the mounting doesn't fail at every attempt, obviously. And when the mount fails, I can fire up DU and it doesn't see the drive(s).
Have you got another good known FW cable you can try and then do some testing?
Or even try using different FW ports if you have any others available, both on the external and your Mac.
PS: I have a FW external drive that would occasionally not mount, and it was either you or another member that discovered the solution which was to do a SMC reset on the Mac. I't worked every time for me, so thanks to whoever provided the info. So just maybe something to try, but it sounds to me that your problem is more like a goofy cable or connection issue.
Just started up the external drive with another failure to mount. Switched cables with no success. Switched FW ports on back of drive with no success. Even connected 2 other external FW drives with no success. So, that, for me, isolates the issue to my MBP. I suppose I can try the SMC reset, although there are absolutely no other issues I'm having with my MBP- just this. And sure enough, a simple restart mounted the drive.
If it happens again, you could first check in System Profiler if the FW drive is present at all. If not, there is definitely a cable or worse ( main board socket) problem.
Except for this: is the drive an old LaCie D2 by any chance? Those have a tendency to weird mounting and sleep behaviour. Try a different setting of the switch on its back ( auto or on).
Have you tried clicking on the greyed out drives in Disk Utility, then clicking on the mount tool?
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I don't remember saying that they're being seen by DU. The only time they will be grayed out is if I unmount them and leave the drive powered up. Then, yes, the drivew will mount again. The problem is when I power the drive down for a period of time. Powering it back up will not mount the drives nor will DU see them.