Removing HD from an array

I have a dedicated server at Hetzner, one of the drives is faulty and although they will replace it I need still need remove the old drive from the array and insert the new drive into the existing array.
On the face of it this is not a difficult job but I've not done it before and I'm having trouble removing the drive from the array. I could continue trying but I fear I'll put in a crap command as I'm dyslexic . So would much prefer it if I could find someone who does this often (I have ten sites on the server).

Anyone with experience of doing this ? here a run down of the problem for anyone interested.

Software RAID:
Personalities : [raid1]
md0 : active raid1 sda1[0] sdb1[1]
      16777088 blocks super 1.0 [2/2] [UU]
     
md3 : active raid1 sdb5[1]
      1839216960 blocks super 1.0 [2/1] [_U]
      bitmap: 11/14 pages [44KB], 65536KB chunk

md2 : active raid1 sdb3[1]
      1073741632 blocks super 1.0 [2/1] [_U]
      bitmap: 6/8 pages [24KB], 65536KB chunk

md1 : active raid1 sda2[0] sdb2[1]
      524224 blocks [2/2] [UU]
     
unused devices:
Partition info:
Filesystem      Size  Used Avail Use% Mounted on
/dev/md2       1008G   20G  937G   3% /
tmpfs            16G     0   16G   0% /dev/shm
/dev/md1        496M   35M  436M   8% /boot
/dev/md3        1.7T  5.3G  1.6T   1% /home



Disk /dev/sdb: 3000.6 GB, 3000592982016 bytes
255 heads, 63 sectors/track, 364801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x798d3974

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1        2089    16777216+  fd  Linux raid autodetect
/dev/sdb2            2089        2155      524288+  fd  Linux raid autodetect
/dev/sdb3            2155      135829  1073741824+  fd  Linux raid autodetect
/dev/sdb4          135830      364802  1839219160    f  W95 Ext'd (LBA)
/dev/sdb5          135830      364802  1839217112+  fd  Linux raid autodetect

Disk /dev/sda: 3000.6 GB, 3000592982016 bytes
255 heads, 63 sectors/track, 364801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0xc93735b1

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1        2089    16777216+  fd  Linux raid autodetect
/dev/sda2            2089        2155      524288+  fd  Linux raid autodetect
/dev/sda3            2155      135829  1073741824+  fd  Linux raid autodetect
/dev/sda4          135830      364802  1839219160    f  W95 Ext'd (LBA)
/dev/sda5          135830      364802  1839217112+  fd  Linux raid autodetect

Disk /dev/md1: 536 MB, 536805376 bytes
2 heads, 4 sectors/track, 131056 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000


Disk /dev/md2: 1099.5 GB, 1099511431168 bytes
2 heads, 4 sectors/track, 268435408 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000


Disk /dev/md3: 1883.4 GB, 1883358167040 bytes
2 heads, 4 sectors/track, 459804240 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000


Disk /dev/md0: 17.2 GB, 17179738112 bytes
2 heads, 4 sectors/track, 4194272 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000

you need to replace sda, if you already have done this then this shouldn't be any problem to do it again.
Also on the internet there are many tutorials for doing that

you need to replace sda, if you already have done this then this shouldn't be any problem to do it again.
Also on the internet there are many tutorials for doing that

Well I was thinking I need to replace sdb !!

I haven't already done it, the last time I hadn't really setup the server so I just got them to change the drive and I started again. This is not the same server but I've only been using it it for 6 weeks.

At the moment I'm finding it difficult to remove the drives from the array.