No blog do Alan encontrei uma dica interessante para monitorar o acesso ao disco

Para ativar
echo 1 > /proc/sys/vm/block_dump

para visualizar:
dmesg
[18761.145648] BrowserBlocking(4512): WRITE block 1536032 on sda5 (32 sectors)
[18761.145654] BrowserBlocking(4512): WRITE block 3633184 on sda5 (32 sectors)
[18761.180958] BrowserBlocking(4512): WRITE block 128 on sda5 (8 sectors)
[18761.200588] btrfs-delalloc-(8064): WRITE block 781894752 on sda5 (8 sectors)
[18761.200826] BrowserBlocking(4512): WRITE block 1536128 on sda5 (32 sectors)
[18761.200833] BrowserBlocking(4512): WRITE block 3633280 on sda5 (32 sectors)
[18761.200889] BrowserBlocking(4512): WRITE block 1536160 on sda5 (32 sectors)