Format an external drive with ext4

I have a Raspberry PI to which an external 1T hard drive is attached by USB. The USB drive works fine if it is formatted in FAT but if I format the drive in ext4 the drive does not mount correctly and it gives me all sorts of error. This is what I do:

fdisk -l

and the result is

Device Boot      Start         End      Blocks   Id  System
/dev/sda1               2  1953525167   976762583   83  Linux

then I run

mkfs.ext4 /dev/sda1

and this runs and does what it does without any erros. then I mount the drive by

mount /dev/sda1 /mnt/external1

and this takes a little while but it returns with no error. Two things happen here. If I do a ls /mnt/external1 I get the following error:

ls: reading directory /mnt/external1: Input/output error

the other thing that happens in that if I do a fdisk -l I will get this now:

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               2  1953525167   976762583   83  Linux

my disk is now moved to sdb1 and if I try to mount sdb1 the same errors happen and the disk reappear under sda1.

What am I doing wrong?

Here is the output from dmesg

[98931.054218] usb 1-1.2: New USB device found, idVendor=13fd, idProduct=1340
[98931.054254] usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[98931.054274] usb 1-1.2: Product: External        
[98931.054290] usb 1-1.2: Manufacturer: Generic 
[98931.054308] usb 1-1.2: SerialNumber: 533144464256454C20202020
[98931.055405] usb-storage 1-1.2:1.0: USB Mass Storage device detected
[98931.058815] scsi12 : usb-storage 1-1.2:1.0
[98932.054219] scsi 12:0:0:0: Direct-Access     Generic  External         2.10 PQ: 0 ANSI: 4
[98932.055913] smsc95xx 1-1.1:1.0 eth0: link up, 100Mbps, full-duplex, lpa 0x45E1
[98932.056737] sd 12:0:0:0: [sdb] 1953525168 512-byte logical blocks: (1.00 TB/931 GiB)
[98932.058233] sd 12:0:0:0: [sdb] Write Protect is off
[98932.058270] sd 12:0:0:0: [sdb] Mode Sense: 21 00 00 00
[98932.059724] sd 12:0:0:0: [sdb] No Caching mode page found
[98932.059763] sd 12:0:0:0: [sdb] Assuming drive cache: write through
[98932.066927] sd 12:0:0:0: [sdb] No Caching mode page found
[98932.066967] sd 12:0:0:0: [sdb] Assuming drive cache: write through
[98932.090896]  sdb: sdb1
[98932.096586] sd 12:0:0:0: [sdb] No Caching mode page found
[98932.096629] sd 12:0:0:0: [sdb] Assuming drive cache: write through
[98932.096654] sd 12:0:0:0: [sdb] Attached SCSI disk
[98935.392602] Buffer I/O error on device sda1, logical block 121667584
[98935.392632] lost page write due to I/O error on sda1
[98935.392651] JBD2: Error -5 detected when updating journal superblock for sda1-8.
[98935.392822] Aborting journal on device sda1-8.
[98935.392857] Buffer I/O error on device sda1, logical block 121667584
[98935.392872] lost page write due to I/O error on sda1
[98935.392887] JBD2: Error -5 detected when updating journal superblock for sda1-8.
[98970.278132] EXT4-fs warning (device sda1): __ext4_read_dirblock:908: error reading directory block (ino 2, block 0)
[98970.278186] EXT4-fs error (device sda1): __ext4_journal_start_sb:62: Detected aborted journal
[98970.300661] EXT4-fs (sda1): Remounting filesystem read-only