rhel6: dd fails reading/writing large blocks

Issue

• When trying to write tapes with large block sizes (1MB+) or reading tapes with large block sizes (10M+), this only succeeds sporadically. The error seen is

Device or resource busy" (read() or write() returns -1, errno = EBUSY)

• We see this:

[root@host]# dd if=<SomeLargeFile> of=/dev/st<x> bs=$((100000))
/bin/dd: writing `/dev/st<x>': Device or resource busy
1+0 records in
0+0 records out
0 bytes (0 B) copied, 20.7455 s, 0.0 kB/s

• Occasionally (1 out of 10) the write() or read() succeeds but mostly (9 out of 10) the operation fails with EBUSY. Smaller Block sizes work flawlessly.