Dell T7910 : Slow POST and after POST black screen of 15s

Hi All,

I happen to have a nice Dell Precison T7910 (dual 12-core Xeon V3's, 160Gb RAM, PERC H730P).

I've owned several Dell Precisions in the recent years and they've all been slow to finish POST but this one has had a variant that worries me:

Here's what happens when I turn on the machine (after a cold shutdown):

- Wait about a minute for the Dell Logo to show up (no worries, my precisions all did that).

- POST starts and displays the H730P Banner (press 'Control-R to..' etc...).

- Then (that's the part that worries me), the screen goes black for about 10-15 seconds.

- ..and then the Grub boot menu gets displayed.

I've reduced by boot list to the bare minimum in an attempt to debug this but to no avail:

- #1 USB Storage

- #2 H730P RAID

What could be causing this lengthy delay?

Ideas would be most welcomed. Thanks.