partitions not getting mounted at boot

[JD]

Ubuntu 12.10 with latest updates.
I have 2 external drives connected to esata2 pcmcia hub.
Lately, what is happening when I reboot, I get a banner
that /dev/sdb1 could not be mounted, press S to skip press M to ????

If press M, andlogin, I run fsck on all the partitions that were supposed
to get mounted (they used to be mounted when I first installed ubuntu 
12.10).
fsck fails and says the device is busy.
Yet it is not mounted at all.
if I ls it's mountmount point as configures in fstab,
no files are visible.
So, why does fsck say it is busy?

This is the case on all partition of both drives.

Now the weird thingis that if I do
mount -a
then all get mounted and everything is fine.
What is causing this weird behaviour??