Greg Cormier wrote:

Is it possible to sleep hard drives in an array?

yes - to be clear you sleep the drives though, not the array.

Can I sleep the drives in my RAID5 array while it's not being used?
It's an XFS partition.

XFS is bad at this IIRC.
Certainly noatime is important to prevent cache accesses from updating the fs.

But I'm fairly sure they are not spinning down :( Is there some
activity mdadm is doing in the background?

from Documentation/laptops/laptop-mode.txt:

If you want to find out which process caused the disk to spin up, you can
gather information by setting the flag /proc/sys/vm/block_dump. When this flag
is set, Linux reports all disk read and write operations that take place, and
all block dirtyings done to files. This makes it possible to debug why a disk
needs to spin up, and to increase battery life even more. The output of
block_dump is written to the kernel output, and it can be retrieved using
"dmesg". When you use block_dump and your kernel logging level also includes
kernel debugging messages, you probably want to turn off klogd, otherwise
the output of block_dump will be logged, causing disk activity that is not
normally there.

also google found
http://www.nslu2-linux.org/wiki/FAQ/HowtoIdentifyWhichProcessesAccessDisk

Before going too far, make sure the array is up but not mounted and ensure that
the drives will actually spin down.

David