access to hard drives is "blocked" by writes to a flash drive

When one of my flash drives is being heavily written to; typically by ``svn update'' on /usr/src, located on the flash drive; the following can be said about filesystem behavior:

- ``svn update'' seems to be able to quickly update a bunch of files, but is then unable to continue for a period of time. This behavior is cyclical, and cycles several times, depending on the amount of updating work to be done for a particular run of ``svn update''.
- Access to any filesystem, whether on the said flash drive or not, seems to be hindered a lot, typically during the said "unable to continue" time. Reading of a file on a hard drive was once delayed for more than 10 seconds. Seemingly as a consequence, the starting of top(1) is also typically delayed.

SU is enabled on all UFS filesystems, including the ones on the hard drives and the flash drive.