I have 4 media servers. Each server has 6 drives.

Two media servers are attached to an older library consisting of LTO gen
1 drives.
The other two are attached to a newer, higher performance LTO gen 2 based
library.

I would like to better load balance jobs between the 4 media servers. However,
certain jobs should be run on the newer library as they are larger/longer
running. Conversely, I have some client machines that can only reach the
older library due to network/zoning issues, so those should be run on the
older library. I don't want jobs to queue up waiting on one library when
drives are available on the other.

My first attempt was to set up Storage Unit Groups that prioritized one library
over the other, so that in 4 groups, each media server was listed first in
each policy. It appears I get more error 52's since doing that, and may
be causing contention between drives. This is anecdotal so this may not
be the root cause.

So what configuration would work best (and without errors) to round robin
drives, with priority, between multiple storage units/media servers?