Installing XenServer 6.5 but does not boot up

Given that none of the supported RAID controllers for the Precision 490 support RAID-10, but apparently you've done something magical with the on-board controller (or Dell are, shockingly, wrong).

The "RAID" controller in the 5000 chipset isn't real hardware RAID, though, so it won't work under Linux. Just use Linux software RAID instead.

An hour of googling was 5 minutes short to find the answer... anyway:

gdisk and sgdisk allow to relocate the second GPT header in "expert mode".

If the basic situation was like this:

# sgdisk -p /dev/sda
Disk /dev/sda: 3902341120 sectors, 1.8 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): 01BFC515-C093-495B-A33F-CA925FB74357
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 1951170526
Partitions will be aligned on 2048-sector boundaries
Total free space is 6042 sectors (3.0 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         8388641   4.0 GiB     0700  
   2         8390656        16777249   4.0 GiB     0700  
   3        16779264      1951170526   922.4 GiB   8E00

(note the last usable sector is 1951170526)

By using the x command in gdisk and then e command, the partition table looks like this:

Expert command (? for help): p
Disk /dev/sda: 3902341120 sectors, 1.8 TiB
 Logical sector size: 512 bytes
Disk identifier (GUID): 01BFC515-C093-495B-A33F-CA925FB74357
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 3902341086
Partitions will be aligned on 2048-sector boundaries
Total free space is 1951176602 sectors (930.4 GiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         8388641   4.0 GiB     0700  
   2         8390656        16777249   4.0 GiB     0700  
   3        16779264      1951170526   922.4 GiB   8E00 

(note the last usable sector is 3902341086)

After that, removing and re-creating a larger partition works without a problem.